In general, a printer is used as an output terminal instrument for an information processing device (hereinafter referred as computer) such as a computer or the like, and is provided with an interface unit (hereinafter referred to as I/F unit) for connecting the printer to the computer. Since there is a variety of computers having different arrangements, the I/F unit mounted on the printer is generally arranged to be compatible for use in a computer of that kind which is normally connected to the printer. To permit the printer to be connected, where required, with another kind of computer, it has been known to provide the printer with a connector for connecting an optional extension I/F unit to the printer. Typically, a conventional connector for the extension I/F unit connection is accommodated in a printer case together with a control printed-circuit board. In this respect, when the extension I/F unit is mounted to the printer, the printer case must be removed from the printer body by the use of tools. This causes inconveniences.
To eliminate such disadvantage, it has been known to permit the extension I/F unit to be mounted, through an opening provided in the printer case, to the connector disposed within the interior of the printer case. However, a considerable difficulty is encountered in mounting the extension I/F unit to the connector through the opening of the printer case. Moreover, printer constituent parts such as electric circuit components disposed within the printer case can be damaged, if the extension I/F unit is brought in contact with the printer parts during the mounting operation.
In this regard, it has been known to form the extension I/F unit into cassette type or cartridge type (see, Japanese Utility Model Provisional Publication Nos. 62-33459, 62-41558 and 62-128845, for instance), so as to permit the extension I/F unit to be mounted without the need of removing the printer case from the printer body. According to this kind of printer, however, the printer case must be arranged to be compatible for use with the extension I/F unit of cassette type or cartridge type. Hence, the printer case must be designed to have a special arrangement suited to the type of the extension I/F unit to be used. This decreases the versatility of the printer case and increases the cost of the printer case.